A family friend of the man who shot and killed 59 people in the deadliest mass shooting in American history has revealed details of the strained relationship between him and his girlfriend - as well as his liking of casino-sponsored prostitutes.

Australian businessman Adam Le Fevre was in a relationship with the sister of Marilou Danley – the partner of Las Vegas gunman Stephen Paddock, who killed 58 others and himself in the massacre on Sunday.

Mr Le Fevre told A Current Affair he went to Las Vegas with Paddock, 64, two years ago with his partner Liza and her sister Marilou, and had travelled to the Philippines with him twice, first in 2013.

He said Paddock was 'condescending' towards Ms Danley, 62, making her 'nervous and jittery', while also making the most of prostitutes offered to him by casinos.

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Adam Le Fevre (pictured) said Las Vegas shooter Stephen Paddock was gun-obsessed and a staunch defender of the right to bear arms. Mr La Fevre was in a relationship with the sister of Marilou Danley, Paddock's girlfriend

He said Paddock (pictured centre) made Ms Danley feel 'nervous and jittery' and spoke to her in a condescending manner

Mr Le Fevre went to Las Vegas with Paddock (pictured centre during a family hotel stay with partner Marilou and her sister Liza) two years ago and had travelled to the Philippines with him twice

Pictured is a drone view of Paddock's house in 2015. The home was raided by police after the massacre that left 59 dead and close to 500 injured

Referring to offers of free prostitutes offered to Paddock by casinos in Las Vegas, Mr Le Fevre said: 'I did have no questions that some of those offers had been accepted, yes'.

He said the relationship between the shooter and his partner was 'not loving' or 'caring'.

'I experienced Steve talking to Marilou in an abrupt manner at times. Marilou... seemed very nervous and jittery around Steve, he would talk in a condescending way at times.'

Mr Le Fervre said he was confident Ms Danley had no knowledge of the planned attack, in which when Paddock opened fire on a music festival from his suite on the 32nd floor of the Mandalay Bay casino before shooting himself dead.

Mr Le Fevre said Paddock had a room full of guns in his home at Mesquite, Nevada (pictured)

Paddock shot dead 58 people and injured more than 500 on Sunday when he opened fire on a music festival

'I think I know Marilou and the family well enough... and all of them a very loving family, and I believe if Marilou had the slightest inkling that something wasn't right, that she would've attempted to intervene and shared that with her family.'

Mr Le Fevre also revealed a photo of the pair taken via a drone, showing the pair in the backyard of Paddock's Nevada property.

'It is a very chilling thought to know that the person that is at the centre of the world's largest story at the moment was somebody that I knew, somebody that I was welcomed into their home, into their family, into their life.'

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Mr Le Fevre said Paddock had a room full of guns in his home at Mesquite and was a stringent defender of the second amendment to the United States constitution.

'He was very strict and very firm on the fact that it's a right. It's the freedom of every American to participate, to own a gun and use it… when need be.'

He said Paddock claimed he earned up to $300,000 a year gambling at Las Vegas casinos, working with a base of roughly one to two million dollars.

Paddock killed 59 people and injured hundreds more in a shooting from a Mandalay Bay hotel room. He broke the windows of his 32nd floor room and opened fire on a music festival crowd

The pair stayed in an 'out-of-this-world penthouse suites like you wouldn't believe' because Paddock was a 'high roller', Le Fevre said.

The girlfriend of Las Vegas mass murderer Stephen Paddock told the FBI he had developed 'mental health symptoms' and would scream at night, it has emerged.

Marilou Danley, 62, said Paddock, 64, 'would lie in bed, just moaning and screaming, "Oh my God,"' according to an ex-FBI official briefed on the situation.

Investigators - who interviewed Danley after she arrived back in the US from the Philippines on Tuesday night - believe that he may have been in 'mental or physical anguish,' that official and another ex-FBI source told NBC News.

The hotel room (pictured) was full of guns and ammunition when entered by authorities

However, they said that detectives are still no closer to determining Paddock's motive for the deranged shooting spree that saw him killing 59 people, including himself, and injuring 489 others.

Ms Danley voluntarily returned to the US in a bid to 'clear her name' of any involvement in the massacre that saw Paddock kill 59 people, including himself, and injure 489 others.

On Wednesday, Ms Danley's lawyer said she believed that Paddock had bought her a plane ticket to the Philippines and then wired $100,000 US dollars because he wanted to break up with her.

He suggested that she visit her family, something she was happy to do, she claimed.

When he wired the large sum of money, she worried that he wanted to end their years-long relationship.

He told 9News went to Las Vegas with Paddock (pictured) two years ago and had travelled to the Philippines with him twice

Danley is an Australian citizen who had renounced her Filipino citizenship, according to a local report by ABS-CBN.

It is not known precisely when she immigrated to Australia.

On her return to the United States she was met by the FBI after she touched down at LAX, and has been described by authorities as a 'person of interest'.

An attorney for Ms Danley insisted she knew nothing about the massacre and had no prior warning.